The ARM-THUMB Procedure Call Standard

The ARM-THUMB Procedure Call Standard5.5 Shared librariesA shared library is a re-entrant program to which other programs may link at execution time. In general, both thelibrary and its client have read-write (RW) segments and the process of linking to a shared library creates a copyof the library’s RW segment for its client.Shared libraries are statically linked independently of one another and independently of their clients. As a result,different values of SB are needed to address the separately linked RW segments in a program that uses one ormore shared libraries. A new value of SB— or a local SB value— must be established when control passes from:ooThe code of the client to the code of a library.The code of a library to the code of a different libraryOn returning, the value of SB must be as it was prior to the call.In the shared-library variant of the ATPCS, a new SB value is established after entry to a shared-library routinethat needs to use SB. The new value need not be established until it is needed and a routine that makes no use ofSB need not establish a new value. Formally:Pre: VAL(SB) = address of any static data segment{called routine}The post-condition VAL(SB) = PRE(SB) is guaranteed by the base standard.5.6 The shared-library data-addressing architectureThe shared-library data addressing architecture extends the RWPI data addressing architecture (see section5.3.1, Position-independent data addressing):ooEach shared library has a unique identity represented by a small integer (the library index) bound into thelibrary code. A library index may be relocated when a library is loaded into RAM, or it may be bound statically.A library’s index is the same in every process threading it.Each (per-process) static data segment begins with a table of four pointers into a process data table. Theypoint to entries 0, 32, 64, and 96 of the process data table, respectively.o The N th entry in a process data table points to the start of the static data segment for library N.In ARM-state, a routine can locate its static data using the code sequence:LDR SB, [SB, #0]LDR SB, [SB, #my_index]; my_index may be relocatedThe base standard guarantees that SB is restored on exit from the subroutine.In Thumb-state, SB is a high register that cannot be used directly so a subroutine can locate its static data using:MOV LSB, SB ; LSB any low registerLDR LSB, [LSB, #my_segment] ; 0, 1, 2, or 3 ...LDR LSB, [LSB, #my_index ; ... and my_index may be relocatedA Thumb-state subroutine does not alter SB so it does not need to restore it.NoteNoteThese code sequences do not need to be inline in their using routines. Each can be replaced by a call toa special (non-ATPCS-conforming) leaf routine, saving some space and reducing the number oflocations dependent on the library index, but costing some execution time.In ARM-state, every non-leaf routine and every static-data-using leaf routine bears the cost of SB’s fixedrole by losing a v-register (but non-static-data-using leaf routines may use v6). In Thumb-state, the localstatic base (LSB) is only needed in static-data-using routines where common sub-expression eliminationand register allocation can be applied to it together with user variables.SWS ESPC 0002 A-05 Page 18 of 37